Operational amplifiers, as is known, have a multitude of uses. For example, differential amplifiers are used in almost all types of communication equipment such as radios, televisions, short wave radios, etc.. The general function of an differential amplifier is to amplify the difference between two input signals. This is generally done using a differential input comprised of a plurality of transistors.
While differential amplifiers perform the multitude of uses very well, there are some inherent difficulties. For example, in differential amplifier, if the transistors are not identically matched, i.e. having the same gain threshold voltage, saturation voltage, etc., an offset will result causing the output to not be a true representation of the difference between the input signals. To combat this difficulty, auto-zeroing circuits have been designed. One such auto-zeroing circuit is disclosed in U.S. Pat. No. 4,710,724, issued to Lawrence Connell, et al.. In the Connell reference, a capacitor is coupled in series with the inputs wherein the capacitor stores an offset voltage due to the transistor mismatching. To establish the voltage across the offset storing capacitors, the inputs are switched between the actual inputs being amplified, or compared, and a reference voltage. When the inputs are coupled to the reference voltage, the load transistors of the amplifier circuit, are coupled to the output such that any offset voltages between the output legs will be absorbed, or stored in the offset storing capacitors.
The differential comparator of Connell, et al. works very well in many applications. However, as portable electronic devices strive for greater battery life, the push for lower voltage applications is a critical issue. Typically, the supply voltage needed by an application is determined by the voltage biasing of the components in series with the supply voltage (Vdd) and ground. The biasing voltages of the prior art are shown in FIG. 1. The biasing is such that the voltage across transistor 4 (Vds) and transistor 6 is equal to a threshold voltage (Vt) and a drain to source saturation voltage (Vdsat) (Vt+Vdsat). The drain to source voltage across transistor 2, which acts as a current source, is a drain to source saturation voltage (Vdsat). The total voltage across these three devices represents the minimum needed supply voltage, and is represented by the equation: EQU Minimum Vdd=Transistor 2 Vds+Transistor 4 Vds+Transistor 6 Vds
Substituting the referenced values: EQU Minimum Vdd=(Vdsat)+(Vt+Vdsat)+(Vt+Vdsat) =2(Vt)+3(Vdsat).
Typically, Vt will have a value of 0.7 V and range from 0.6 V to 0.8 V. While, Vdsat will have a typical value of 0.15 V and a range from 0.1 V to 0.2 V. Substituting these values into the above equation shows that a Vdd of 1.85 V will be needed in a typical situation. In a worse case situation a Vdd of 2.2 V is needed, while, in a best case situation a Vdd of 1.5 V will be needed. The prior art requires a Vdd of between 1.5 V and 2.2 V. Therefore, a need exists for a differential amplifier, or comparator, circuit that operates at or below a voltage source that is 1.5 V.